Formation of the retinal ganglion cell and optic fiber layers

Single cell transcriptome profiling of retinal ganglion cells identifies cellular subtypes

Retinal ganglion cells (RGCs) convey the major output of information collected from the eye to the brain. Thirty subtypes of RGCs have been identified to date. Here, we analyze 6225 RGCs (average of 5000 genes per cell) from right and left eyes by single-cell RNA-seq and classify them into 40 subtypes using clustering algorithms. We identify additional subtypes and markers, as well as transcription factors predicted to cooperate in specifying RGC subtypes. Zic1, a marker of the right eye-enriched subtype, is validated by immunostaining in situ. Runx1 and Fst, the markers of other subtypes, are validated in purified RGCs by fluorescent in situ hybridization (FISH) and immunostaining. We show the extent of gene expression variability needed for subtype segregation, and we show a hierarchy in diversification from a cell-type population to subtypes. Finally, we present a website for comparing the gene expression of RGC subtypes.

The Beta Adrenergic Receptor Blocker Propranolol Counteracts Retinal Dysfunction in a Mouse Model of Oxygen Induced Retinopathy: Restoring the Balance between Apoptosis and Autophagy

In a mouse model of oxygen induced retinopathy (OIR), beta adrenergic receptor (BAR) blockade has been shown to recover hypoxia-associated retinal damages. Although the adrenergic signaling is an important regulator of apoptotic and autophagic processes, the role of BARs in retinal cell death remains to be elucidated. The present study was aimed at investigating whether ameliorative effects of BAR blockers may occur through their coordinated action on apoptosis and autophagy. To this aim, retinas from control and OIR mice untreated or treated with propranolol, a non-selective BAR1/2 blocker, were characterized in terms of expression and localization of apoptosis and autophagy markers. The effects of propranolol on autophagy signaling were also evaluated and specific autophagy modulators were used to get functional information on the autophagic effects of BAR antagonism. Finally, propranolol effects on neurodegenerative processes were associated to an electrophysiological investigation of retinal function by recording electroretinogram (ERG). We found that retinas of OIR mice are characterized by increased apoptosis and decreased autophagy, while propranolol reduces apoptosis and stimulates autophagy. In particular, propranolol triggers autophagosome formation in bipolar, amacrine and ganglion cells that are committed to die by apoptosis in response to hypoxia. Also our data argue that propranolol, through the inhibition of the Akt-mammalian target of rapamycin pathway, activates autophagy which decreases retinal cell death. At the functional level, propranolol recovers dysfunctional ERG by recovering the amplitude of a- and b-waves, and oscillatory potentials, thus indicating an efficient restoring of retinal transduction. Overall, our results demonstrate that BAR1/2 are key regulators of retinal apoptosis/autophagy, and that BAR1/2 blockade leads to autophagy-mediated neuroprotection. Reinstating the balance between apoptotic and autophagic machines may therefore be viewed as a future goal in the treatment of retinopathies.

Autophagy-mediated neuroprotection induced by octreotide in an ex vivo model of early diabetic retinopathy

Neuronal injury plays a major role in diabetic retinopathy (DR). Our hypothesis was that the balance between neuronal death and survival may depend on a similar equilibrium between apoptosis and autophagy and that a neuroprotectant may act by influencing this equilibrium. Ex vivo mouse retinal explants were treated with high glucose (HG) for 10days and the somatostatin analog octreotide (OCT) was used as a neuroprotectant. Chloroquine (CQ) was used as an autophagy inhibitor. Apoptotic and autophagic markers were evaluated using western blot and immunohistochemistry. HG-treated explants displayed a significant increase of apoptosis paralleled by a significant decrease of the autophagic flux, which was likely to be due to increased activity of the autophagy regulator mTOR (mammalian target of rapamycin). Treatment with OCT rescued HG-treated retinal explants from apoptosis and determined an increase of autophagic activity with concomitant mTOR inhibition. Blocking the autophagic flux with CQ completely abolished the anti-apoptotic effect of OCT. Immunohistochemical observations showed that OCT-induced autophagy is localized to populations of bipolar and amacrine cells and to ganglion cells. These observations revealed the antithetic role of apoptosis and autophagy, highlighting their equilibrium from which neuronal survival is likely to depend. These data suggest the crucial role covered by autophagy, which could be considered as a molecular target for DR neuroprotective treatment strategies.

Intermittent high oxygen influences the formation of neural retinal tissue from human embryonic stem cells

The vertebrate retina is a highly multilayered nervous tissue with a large diversity of cellular components. With the development of stem cell technologies, human retinas can be generated in three-dimensional (3-D) culture in vitro. However, understanding the factors modulating key productive processes and the way that they influence development are far from clear. Oxygen, as the most essential element participating in metabolism, is a critical factor regulating organic development. In this study, using 3-D culture of human stem cells, we examined the effect of intermittent high oxygen treatment (40% O₂) on the formation and cellular behavior of neural retinas (NR) in the embryonic body (EB). The volume of EB and number of proliferating cells increased significantly under 40% O₂ on day 38, 50, and 62. Additionally, the ratio of PAX6+ cells within NR was significantly increased. The neural rosettes could only develop with correct apical-basal polarity under 40% O₂. In addition, the generation, migration and maturation of retinal ganglion cells were enhanced under 40% O₂. All of these results illustrated that 40% O₂ strengthened the formation of NR in EB with characteristics similar to the in vivo state, suggesting that the hyperoxic state facilitated the retinal development in vitro.

Anatomical identification of extracellularly recorded cells in large-scale multielectrode recordings

This study combines for the first time two major approaches to understanding the function and structure of neural circuits: large-scale multielectrode recordings, and confocal imaging of labeled neurons. To achieve this end, we develop a novel approach to the central problem of anatomically identifying recorded cells, based on the electrical image: the spatiotemporal pattern of voltage deflections induced by spikes on a large-scale, high-density multielectrode array. Recordings were performed from identified ganglion cell types in the macaque retina. Anatomical images of cells in the same preparation were obtained using virally transfected fluorescent labeling or by immunolabeling after fixation. The electrical image was then used to locate recorded cell somas, axon initial segments, and axon trajectories, and these signatures were used to identify recorded cells. Comparison of anatomical and physiological measurements permitted visualization and physiological characterization of numerically dominant ganglion cell types with high efficiency in a single preparation.

p27(Kip1) participates in the regulation of endoreplication in differentiating chick retinal ganglion cells

Nuclear DNA duplication in the absence of cell division (i.e. endoreplication) leads to somatic polyploidy in eukaryotic cells. In contrast to some invertebrate neurons, whose nuclei may contain up to 200,000-fold the normal haploid DNA amount (C), polyploid neurons in higher vertebrates show only 4C DNA content. To explore the mechanism that prevents extra rounds of DNA synthesis in these latter cells we focused on the chick retina, where a population of tetraploid retinal ganglion cells (RGCs) has been described. We show that differentiating chick RGCs that express the neurotrophic receptors p75 and TrkB while lacking retinoblastoma protein, a feature of tetraploid RGCs, also express p27^Kip1. Two different short hairpin RNAs (shRNA) that significantly downregulate p27^Kip1 expression facilitated DNA synthesis and increased ploidy in isolated chick RGCs. Moreover, this forced DNA synthesis could not be prevented by Cdk4/6 inhibition, thus suggesting that it is triggered by a mechanism similar to endoreplication. In contrast, p27^Kip1 deficiency in mouse RGCs does not lead to increased ploidy despite previous observations have shown ectopic DNA synthesis in RGCs from p27^Kip1−/− mice. This suggests that a differential mechanism is used for the regulation of neuronal endoreplication in mammalian versus avian RGCs.

Deficiency of aldose reductase attenuates inner retinal neuronal changes in a mouse model of retinopathy of prematurity

Retinopathy of prematurity (ROP) is a leading cause of childhood blindness where vascular abnormality and retinal dysfunction are reported. We showed earlier that genetic deletion of aldose reductase (AR), the rate-limiting enzyme in the polyol pathway, reduced the neovascularization through attenuating oxidative stress induction in the mouse oxygen-induced retinopathy (OIR) modeling ROP. In this study, we further investigated the effects of AR deficiency on retinal neurons in the mouse OIR. Seven-day-old wild-type and AR-deficient mice were exposed to 75 % oxygen for 5 days and then returned to room air. Electroretinography was used to assess the neuronal function at postnatal day (P) 30. On P17 and P30, retinal cytoarchitecture was examined by morphometric analysis and immunohistochemistry for calbindin, protein kinase C alpha, calretinin, Tuj1, and glial fibrillary acidic protein. In OIR, attenuated amplitudes and delayed implicit time of a-wave, b-wave, and oscillatory potentials were observed in wild-type mice, but they were not significantly changed in AR-deficient mice. The morphological changes of horizontal, rod bipolar, and amacrine cells were shown in wild-type mice and these changes were partly preserved with AR deficiency. AR deficiency attenuated the Müller cell gliosis induced in OIR. Our observations demonstrated AR deficiency preserved retinal functions in OIR and AR deficiency could partly reduce the extent of retinal neuronal histopathology. These findings suggested a therapeutic potential of AR inhibition in ROP treatment with beneficial effects on the retinal neurons.

Repurposing Reelin: the new role of radial glia, Reelin and Notch in motor neuron migration

The role of Reelin during cerebral cortical neuron migration has long been studied, but the Reelin signaling pathway and its possible interactions are just beginning to be unraveled. Reelin is not only important in cerebral cortical migration, but has recently been shown to interact with the Notch signaling pathway and to be critical for radial glial cell number and morphology. Lee and Song (2013) show a new Notch- and Reelin-dependent role for radial glia in the mouse spinal cord: to act as a fine filter that allows somatic motor neuron axons but not cell bodies to traverse out of the CNS. Here, the types of neuronal migration are discussed, focusing on motor neurons and cues for proper localization. The interaction of Reelin signaling with the Notch pathway is reviewed, which dictates the proper formation of radial glia in the spinal cord in order to prevent ectopic motor neuron migration (Lee and Song, 2013). Future studies may reveal novel interactions and further insights as to how Reelin functions throughout the developing nervous system.

ADAM10 mediates N-cadherin ectodomain shedding during retinal ganglion cell differentiation in primary cultured retinal cells from the developing chick retina

Here, we examined the role of ADAM10 during retinal cell differentiation in retinal sections and in vitro cultures of developing chick retinal cells from embryonic day 6 (ED6). Immunohistochemistry showed that ADAM10 is abundantly expressed in the inner zone of neuroblastic layer at ED5, and it becomes more highly expressed in the ganglion cell layer at ED7 and ED9. Western blotting confirmed that ADAM10 was expressed as an inactive pro-form that was processed to a shorter, active form in control cultured cells, but in cultures treated with an ADAM10 inhibitor (GI254023X) and ADAM10-specific siRNA, the level of mature ADAM10 decreased. Phase-contrast microscopy showed that long neurite extensions were present in untreated cultures 24 h after plating, whereas cultures treated with GI254023X showed significant decreases in neurite extension. Immunofluorescence staining revealed that there were far fewer differentiated ganglion cells in ADAM10 siRNA and GI254023X-treated cultures compared to controls, whereas the photoreceptor cells were unaltered. The Pax6 protein was more strongly detected in the differentiated ganglion cells of control cultures compared to ADAM10 siRNA and GI254023X-treated cultures. N-cadherin ectodomain shedding was apparent in control cultures after 24 h, when ganglion cell differentiation was observed, but ADAM10 siRNA and GI254023X treatment inhibited these processes. In contrast, N-cadherin staining was strongly detected in photoreceptor cells regardless of ADAM10 siRNA and GI254023X treatment. Taken together, these data indicate that the inhibition of ADAM10 can inhibit Pax6 expression and N-cadherin ectodomain shedding in retinal cells, possibly affecting neurite outgrowth and ganglion cell differentiation.

Brain-derived neurotrophic factor-dependent cdk1 inhibition prevents G2/M progression in differentiating tetraploid neurons

Neurodegeneration is often associated with DNA synthesis in neurons, the latter usually remaining for a long time as tetraploid cells before dying by apoptosis. The molecular mechanism preventing G2/M transition in these neurons remains unknown, but it may be reminiscent of the mechanism that maintains tetraploid retinal ganglion cells (RGCs) in a G2-like state during normal development, thus preventing their death. Here we show that this latter process, known to depend on brain-derived neurotrophic factor (BDNF), requires the inhibition of cdk1 by TrkB. We demonstrate that a subpopulation of chick RGCs previously shown to become tetraploid co-expresses TrkB and cdk1 in vivo. By using an in vitro system that recapitulates differentiation and cell cycle re-entry of chick retinal neurons we show that BDNF, employed at concentrations specific for the TrkB receptor, reduces the expression of cdk1 in TrkB-positive, differentiating neurons. In this system, BDNF also inhibits the activity of both endogenous cdk1 and exogenously-expressed cdk1/cyclin B1 complex. This inhibition correlates with the phosphorylation of cdk1 at Tyr15, an effect that can be prevented with K252a, a tyrosine kinase inhibitor commonly used to prevent the activity of neurotrophins through their Trk receptors. The effect of BDNF on cdk1 activity is Tyr15-specific since BDNF cannot prevent the activity of a constitutively active form of cdk1 (Tyr15Phe) when expressed in differentiating retinal neurons. We also show that BDNF-dependent phosphorylation of cdk1 at Tyr15 could not be blocked with MK-1775, a Wee1-selective inhibitor, indicating that Tyr15 phosphorylation in cdk1 does not seem to occur through the canonical mechanism observed in proliferating cells. We conclude that the inhibition of both expression and activity of cdk1 through a BDNF-dependent mechanism contributes to the maintenance of tetraploid RGCs in a G2-like state.

Ectopic vesicular glutamate release at the optic nerve head and axon loss in mouse experimental glaucoma

Although clinical and experimental observations indicate that the optic nerve head (ONH) is a major site of axon degeneration in glaucoma, the mechanisms by which local retinal ganglion cell (RGC) axons are injured and damage spreads among axons remain poorly defined. Using a laser-induced ocular hypertension (LIOH) mouse model of glaucoma, we found that within 48 h of intraocular pressure elevation, RGC axon segments within the ONH exhibited ectopic accumulation and colocalization of multiple components of the glutamatergic presynaptic machinery including the vesicular glutamate transporter VGLUT2, several synaptic vesicle marker proteins, glutamate, the soluble N-ethylmaleimide-sensitive factor attachment protein receptor complex and active zone cytomatrix components, as well as ultrastructurally identified, synaptophysin-containing vesicles. Ectopic vesicle exocytosis and glutamate release were detected in acute preparations of the LIOH ONH. Immunolocalization and analysis using the ionotropic receptor channel-permeant cation agmatine indicated that ONH axon segments and glia expressed glutamate receptors, and these receptors were more active after LIOH compared with controls. Pharmacological antagonism of glutamate receptors and neuronal activity resulted in increased RGC axon sparing in vivo. Furthermore, in vivo RGC-specific genetic disruption of the vesicular glutamate transporter VGLUT2 or the obligatory NMDA receptor subunit NR1 promoted axon survival in experimental glaucoma. As the inhibition of ectopic glutamate vesicular release or glutamate receptivity can independently modify the severity of RGC axon loss, synaptic release mechanisms may provide useful therapeutic entry points into glaucomatous axon degeneration.

Math5 defines the ganglion cell competence state in a subpopulation of retinal progenitor cells exiting the cell cycle

The basic helix-loop-helix (bHLH) transcription factor Math5 (Atoh7) is transiently expressed during early retinal histogenesis and is necessary for retinal ganglion cell (RGC) development. Using nucleoside pulse-chase experiments and clonal analysis, we determined that progenitor cells activate Math5 during or after the terminal division, with progressively later onset as histogenesis proceeds. We have traced the lineage of Math5+ cells using mouse BAC transgenes that express Cre recombinase under strict regulatory control. Quantitative analysis showed that Math5+ progenitors express equivalent levels of Math5 and contribute to every major cell type in the adult retina, but are heavily skewed toward early fates. The Math5>Cre transgene labels 3% of cells in adult retina, including 55% of RGCs. Only 11% of Math5+ progenitors develop into RGCs; the majority become photoreceptors. The fate bias of the Math5 cohort, inferred from the ratio of cone and rod births, changes over time, in parallel with the remaining neurogenic population. Comparable results were obtained using Math5 mutant mice, except that ganglion cells were essentially absent, and late fates were overrepresented within the lineage. We identified Math5-independent RGC precursors in the earliest born (embryonic day 11) retinal cohort, but these precursors require Math5-expressing cells for differentiation. Math5 thus acts permissively to establish RGC competence within a subset of progenitors, but is not sufficient for fate specification. It does not autonomously promote or suppress the determination of non-RGC fates. These data are consistent with progressive and temporal restriction models for retinal neurogenesis, in which environmental factors influence the final histotypic choice.

Retinal histogenesis and cell differentiation in an elasmobranch species, the small-spotted catshark Scyliorhinus canicula

Here we present a detailed study of the major events in the retinal histogenesis in a slow-developing elasmobranch species, the small-spotted catshark, during embryonic, postnatal and adult stages using classical histological and immunohistological methods, providing a complete neurochemical characterization of retinal cells. We found that the retina of the small-spotted catshark was fully differentiated prior to birth. The major developmental events in retinal cell differentiation occurred during the second third of the embryonic period. Maturational features described in the present study were first detected in the central retina and, as development progressed, they spread to the rest of the retina following a central-to-peripheral gradient. While the formation of both plexiform layers occurs simultaneously in the retina of the most common fish models, in the small-spotted catshark retina the emergence of the outer plexiform layer was delayed with respect to the inner plexiform layer. According to the expression of the markers used, retinal cell differentiation followed a vitreal-to-scleral gradient, with the exception of Müller cells that were the last cell type generated during retinogenesis. This vitreal-to-scleral progression of neural differentiation seems to be specific to slow-developing fish species.

Involvement of EphB/Ephrin-B signaling in axonal survival in mouse experimental glaucoma

PURPOSE

To examine the functional significance of EphB/ephrin-B upregulation in mouse experimental glaucoma.

METHODS

In a loss-of-function approach, mouse mutants lacking EphB2 (EphB2(-/-)) or EphB3 (EphB3(-/-)) protein, and mutants expressing EphB2 truncated in the C-terminus (EphB2(lacZ/lacZ)) were subjected to laser-induced ocular hypertension (LIOH), an experimental mouse model of glaucoma. The number of optic nerve axons was counted in paraphenylenediamine (PPD)-stained sections and compared between EphB mutants and wild type littermates. In a gain-of-function approach, retina/optic nerve explants obtained from LIOH-treated animals were exposed to EphB2-Fc recombinant proteins or Fc control proteins. Tissue sections through the optic nerve head (ONH) were labeled with neuron-specific anti-tubulin β-III antibody to determine axonal integrity.

RESULTS

Both EphB2 and EphB3 null mutant mice exhibited more severe axonal degeneration than wild type littermates after treatment with LIOH. Mutant mice in which the C-terminal portion of EphB2 is truncated had an intermediate phenotype. Application of EphB2-Fc recombinant protein to LIOH-treated optic nerve explants resulted in greater sparing of tubulin β-III-containing retinal ganglion cell (RGC) axons.

CONCLUSIONS

These results provide genetic evidence in mice that both EphB/ephrin-B forward and reverse signaling feed into an endogenous pathway to moderate the effects of glaucomatous insult on RGC axons. LIOH-induced axon loss is maintained in retina/optic nerve explants after removal from an ocular hypertensive environment. Exogenous application of EphB2 protein enhances RGC axon survival in explants, suggesting that modulation of Eph/ephrin signaling may be of therapeutic interest.

Differential expression of the ADAMs in developing chicken retina

The expression patterns of the seven members of the ADAM (a disintegrin and metalloprotease) family, ADAM9, ADAM10, ADAM12, ADAM13, ADAM17, ADAM22, and ADAM23 were analyzed in the developing chicken retina by in situ hybridization and immunohistochemistry. Results show that each individual ADAM is expressed and regulated spatiotemporally in the developing retinal layers. ADAM9, ADAM10 and ADAM17 are widely expressed in the differential layers of the retina throughout the whole embryonic period, while ADAM12 and ADAM13 are mainly expressed in the ganglion cell layer at a later stage. ADAM22 and ADAM23 are restricted to the inner nuclear layer and the ganglion cell layer at a later stage. Furthermore, ADAM10 protein is co-expressed with the four members of the classic cadherins, N-cadherin, R-cadherin, cadherin-6B and cadherin-7 in distinct retinal layers. Therefore, the differential expression of the investigated ADAMs in the developing retina suggests the contribution of them to the retina development.

In vivo development of dendritic orientation in wild-type and mislocalized retinal ganglion cells

Background

Many neurons in the central nervous system, including retinal ganglion cells (RGCs), possess asymmetric dendritic arbors oriented toward their presynaptic partners. How such dendritic arbors become biased during development in vivo is not well understood. Dendritic arbors may become oriented by directed outgrowth or by reorganization of an initially unbiased arbor. To distinguish between these possibilities, we imaged the dynamic behavior of zebrafish RGC dendrites during development in vivo. We then addressed how cell positioning within the retina, altered in heart-and-soul (has) mutants, affects RGC dendritic orientation.

Results

In vivo multiphoton time-lapse analysis revealed that RGC dendrites initially exhibit exploratory behavior in multiple directions but progressively become apically oriented. The lifetimes of basal and apical dendrites were generally comparable before and during the period when arbors became biased. However, with maturation, the addition and extension rates of basal dendrites were slower than those of the apical dendrites. Oriented dendritic arbors were also found in misplaced RGCs of the has retina but there was no preferred orientation amongst the population. However, has RGCs always projected dendrites toward nearby neuropil where amacrine and bipolar cell neurites also terminated. Chimera analysis showed that the abnormal dendritic organization of RGCs in the mutant was non-cell autonomous.

Conclusions

Our observations show that RGC dendritic arbors acquire an apical orientation by selective and gradual restriction of dendrite addition to the apical side of the cell body, rather than by preferential dendrite stabilization or elimination. A biased arbor emerges at a stage when many of the dendritic processes still appear exploratory. The generation of an oriented RGC dendritic arbor is likely to be determined by cell-extrinsic cues. Such cues are unlikely to be localized to the basal lamina of the inner retina, but rather may be provided by cells presynaptic to the RGCs.

Serotonergic neurons migrate radially through the neuroepithelium by dynamin-mediated somal translocation

Embryonic CNS neurons can migrate from the ventricular zone to their final destination by radial glial-guided locomotion. Another less appreciated mechanism is somal translocation, where the young neuron maintains its primitive ventricular and pial processes, through which the cell body moves. A major problem in studying translocation has been the identification of neuronal-specific markers that appear in primitive, radially shaped cells. We used enhanced yellow fluorescent protein under control of the Pet-1 enhancer/promoter region (ePet-EYFP), a specific marker of early differentiated serotonergic neurons, to study their migration via immunohistology and time-lapse imaging of living slice cultures. As early as E10.0, ePet-EYFP-expressing neurons were axonless, radially oriented, and spanned the entire neuroepithelium. The soma translocated within the pial process toward the pial surface and could also translocate through its neurites, which sprouted from the pial process. The dynamin inhibitor dynasore significantly reduced translocation velocity, while the nonmuscle myosin II inhibitor blebbistatin and the kinesin inhibitor AMP-PNP had no significant effect. Here we show for the first time that serotonergic neurons migrate by somal translocation mediated, in part, by dynamin.

Somatic tetraploidy in specific chick retinal ganglion cells induced by nerve growth factor

A subset of neurons in the normal vertebrate nervous system contains double the normal amount of DNA in their nuclei. These neurons are all thought to derive from aberrant mitoses in neuronal precursor cells. Here we show that endogenous NGF induces DNA replication in a subpopulation of differentiating chick retinal ganglion cells that express both the neurotrophin receptor p75 and the E2F1 transcription factor, but that lack the retinoblastoma protein. Many of these neurons avoid G2/M transition and remain alive in the retina as tetraploid cells with large cell somas and extensive dendritic trees, and most of them express beta2 nicotinic acetylcholine receptor subunits, a specific marker of retinal ganglion cells innervating lamina F in the stratum-griseum-et-fibrosum-superficiale of the tectal cortex. Tetraploid neurons were also observed in the adult mouse retina. Thus, a developmental program leading to somatic tetraploidy in specific retinal neurons exists in vertebrates. This program might occur in other vertebrate neurons during normal or pathological situations.

Developmental control of synaptic receptivity

Are neurons born with the ability to form and receive synapses or do they acquire these abilities during development? We have previously found that purified postnatal retinal ganglion cells (RGCs) require soluble astrocyte-derived signals to form synapses in vitro and in vivo. Here we show that newly generated embryonic day 17 (E17) RGCs are able to form but not receive synapses under these conditions. Dendrite growth is not sufficient to trigger receptivity; rather, the ability of newly generated RGCs to receive synapses is acquired at E19 in response to direct contact by neighboring cell types. Direct contact with astrocytes, which are not present at E17 but are normally generated by E19, is sufficient to induce synaptic receptivity in E17 RGCs. In contrast, amacrine contact does not induce synaptic receptivity. Interestingly, astrocyte contact alters the localization of the synaptic adhesion molecule neurexin away from dendrites. In addition, dendritic expression of neurexin is sufficient to prevent astrocyte contact-mediated increases in synapse number, suggesting a molecular mechanism by which astrocyte contact regulates neuronal synaptic receptivity. Thus, synaptic receptivity is not induced simply by dendritic elaboration but must be signaled by both contact-mediated signaling from astrocytes and a shift in the dendritic localization of neurexin.

Epigallocatechin gallate, an active ingredient from green tea, attenuates damaging influences to the retina caused by ischemia/reperfusion

The aim of this study was to examine whether the antioxidant epigallocatechin gallate (EGCG), a catechin-base flavonoid derived from green tea protects retina neurones in situ from ischemia/reperfusion and in vitro from an oxidative stress insult of hydrogen peroxide (H(2)O(2)). Similar results were obtained when rats were injected by two different regimes of EGCG. Ischemia was delivered by raising the intraocular pressure above the systolic blood pressure (120 mm Hg) generally for 45 min. The electroretinogram (ERG) was measured prior to ischemia and 5 days after reperfusion. Rats were killed 7 days after ischemia and processed for immunohistochemistry and for determining of mRNA and protein levels by RT-PCR and electrophoresis/western blotting, respectively. In addition, optic nerves 7 days after ischemia were subjected to protein analysis. Ischemia/reperfusion caused a significant reduction in the a- and b-wave amplitudes of the ERGs, a decrease in retinal ganglion cell and photoreceptor specific proteins and mRNAs, an increase in retinal caspase-3 mRNA and protein, an increase in retinal caspase-8 mRNA, an increase in retinal GFAP protein and mRNA and a decrease in optic nerve proteins associated with ganglion cell axons. All these changes were significantly counteracted by EGCG. Moreover, EGCG clearly blunted ischemia/reperfusion-induced changes in the localisation of retinal Thy-1 and ChAT immunoreactivities. EGCG also significantly reduced the apoptosis to retinal ganglion cells (RGC-5 cells) in culture caused by H(2)O(2). The results of the study demonstrate that EGCG provides protection to retinal neurones from oxidative stress and ischemia/reperfusion.

In vivo reactivation of a quiescent cell population located in the ocular ciliary body of adult mammals

Rare quiescent cells with stem cell characteristics have been isolated from the ocular ciliary body (CB) of adult mammals. In vitro, adult retinal stem cells were reported to generate sphere colonies containing multipotent retinal progenitor cells. Whether proliferation of this stem cell population can be stimulated in vivo in order to generate new retinal cells is an important issue. Herein we report on the in vivo reactivation of a quiescent cell population present in the CB upon growth factors (GF) stimulation. GF stimulation resulted in the re-acquisition of embryonic characteristics (Nestin) and expression of the cell cycle entry markers CyclinD1 and Ki67 by a subset of CB epithelial cells. This inductive effect was not observed in the neural retina. GF-activated CB epithelial cells co-express the retinal progenitor homeodomain transcription factors Pax6 and Chx10. Serial GF injections led to do novo proliferation of clusters of cells in the CB, in a dose-dependent manner, as revealed by bromodeoxyuridine (BrdU) incorporation. Analysis of cells' BrdU content within individual clusters suggests a mode of cell division that is predominantly asymmetric. Cell proliferation was not induced by CB or retinal damage, as indicated by the absence of TUNEL-labeled cells. Newly produced cells did not migrate into the retina nor did they differentiate into retinal neurons. This study demonstrates that proliferation of a quiescent cell population with retinal stem/progenitor cell characteristics can be reactivated in vivo upon GF injections and suggests that, in adult mammals, the CB is a non-permissive environment for cell migration and neurogenesis.

Multiple requirements for Hes 1 during early eye formation

During embryogenesis, multiple developmental processes are integrated through their precise temporal regulation. Hes1 is a transcriptional repressor that regulates the timing of mammalian retinal neurogenesis. However, roles for Hes1 in early eye development have not been well defined. Here, we show that Hes1 is expressed in the forming lens, optic vesicle, cup, and pigmented epithelium and is necessary for proper growth, morphogenesis, and differentiation of these tissues. Because Hes1 is required throughout the eye, we investigated its interaction with Pax6. Hes1-Pax6 double mutant embryos are eyeless suggesting these genes are coordinately required for initial morphogenesis and outgrowth of the optic vesicle. In Hes1 mutants, Math5 expression is precocious along with retinal ganglion cell, amacrine, and horizontal neuron formation. In contrast to apparent cooperativity between Pax6 and Hes1 during morphogenesis, each gene regulates Math5 and RGC genesis independently. Together, these studies demonstrate that Hes1, like Pax6, simultaneously regulates multiple developmental processes during optic development.

Emergence of realistic retinal networks in culture promoted by the superior colliculus

The developing retina is characterized by 'retinal waves', spontaneous depolarizations that propagate through a developing network of interneurons and retinal ganglion cells. Although the circuitry underlying retinal waves is well characterized, the secreted factors that are critical for its normal development are not defined. Dissociated cell culture provides an ideal system for defining these factors; however, it is difficult to recapitulate retinal circuitry in culture. Here we demonstrate that by culturing dissociated retinal neurons in the presence of cells from the superior colliculus (SC), retinal neurons form networks that are similar to those described in the intact retina. Whole-cell voltage clamp recordings reveal the presence of a spontaneously active network of interneurons. In addition, we observed spontaneous, propagating activity reminiscent of that observed in the intact retina. We propose that the presence of factors secreted from the SC results in the development of networks that reproduce critical features of the intact retina.

Characteristics of cellular proliferation in the developing human retina

The expression of proliferation-associated proteins Ki67 and PCNA was studied in the retinal rudiments of human embryos at 5-8 weeks of development; studies also addressed the numbers of nucleoli in the nuclei of neuroepithelial cells (with consideration of their distances to the apical surface) and DNA-synthesizing cells after transient (20 min) in vitro incubation in serum-free medium containing BrdU. The retinal rudiment of embryos at five weeks of development had neuroepithelium of the typical structure. BrdU-positive nuclei and nuclei with small numbers of nucleoli were located in the basal part of the ventricular zone. However, this organization was disrupted during the initial period of formation of the inner nuclear layer (six weeks). At this time, DNA-synthesizing cells were found even at the apical surface. Retinal rudiments of embryos at 6-7 weeks of development contained an additional area of cell proliferation in the Chievitz layer and the inner nuclear layer. In eight-week embryos, dividing cells were located in the outer nuclear layer, which again acquired the organization typical of neuroepithelium.

Growth factors induce neurogenesis in the ciliary body

The ciliary body of the eye is a nonneural tissue that is derived from the anterior rim of the optic cup, an extension of the neural tube. This tissue normally does not contain neurons and functions to produce components of the aqueous humor. We found that intraocular injections of insulin, EGF, or FGF2 stimulate NPE cells to proliferate and differentiate into neurons. These growth factors had region-specific effects along the radial axis of the ciliary body, with insulin and EGF stimulating proliferation of NPE cells close to the retina, while FGF2 stimulated the proliferation of NPE cells further toward the lens. Similar region-specific effects were observed for accumulations of neurons in the NPE in response to injections of different growth factors. The neurons derived from NPE cells express neurofilament, beta3 tubulin, RA4, calretinin, Islet1, or Hu, and a few produced long axonal projections, several millimeters in length that extend across the ciliary body. Our results suggest that the ciliary body has the capacity to generate retinal neurons, but normally neurogenesis is actively inhibited.

Precursors of neurons, neuroglia, and ependymal cells in the CNS: what are they? Where are they from? How do they get where they are going?

Neurons, neuroglia (astrocytes and oligodendrocytes), and ependymal cells are three distinct categories of neural cells in the central nervous system. In the mature brain and spinal cord, the classical histological criteria define these cells by their microscopic structure very well. During development, the precursors for all of these cells reside within the epithelium of the neural plate and its successor, the neural tube. These precursor cells are the undifferentiated, primitive neuroepithelium of the classical literature. As the cerebral vesicles enlarge and their walls thicken, the primitive neuroepithelial cells elongate, maintaining a radial orientation until they migrate. Although many, but not all, of these cells span the extent of the ventricular wall, they are the precursors of neurons, neuroglia, and ependymal cells. Thus, it is useful to retain their classical designation as primitive neuroepithelial cells and to treat them as neural precursor cells. Neural precursor cells are neither neuroglia nor neurons. It is not appropriate to call them radial glial cells anymore than it is to call them radial neuronal cells. The term "radial glia" has long been used to describe the mature, elongated astrocytes, represented by Bergmann cells in the cerebellum and Müller cells in the retina. Inevitably, during development, transitional forms between neural precursor cells and the neurons, neuroglia, and ependymal cells will occur. Such transitional cells are known as neuroblasts, glioblasts, or ependymoblasts, even though they may be postmitotic. Alternative terms are "immature neurons," "immature neuroglia," and "immature ependymal cells." The migration of many neural precursor cells is accomplished by translocation rather than free cellular locomotion. There is both direct and indirect evidence to document the translocation of the nuclear/perikaryal/somal complex through the leading process of primitive neuroepithelial cells. This is conspicuous in the neocortex, where the discrete radial arrangement of pyramidal cells may result from translocation of neuroblasts, while their leading processes still contact the pial surface. Migration by translocation occurs throughout the CNS. GLIA 43:6-18, 2003.

Morphological asymmetry in dividing retinal progenitor cells

For the understanding of histogenetic events in the 3-D retinal neuroepithelium, direct observation of the progenitor cells and their morphological changes is required. A slice culture method has been developed by which the behavior of single progenitor cells can be monitored. Although it has been believed that each retinal progenitor cell loses its basal process while it is in M phase, it is reported here that the process is retained throughout M phase and is inherited by one daughter cell, which can be a neuron or a progenitor cell. Daughter neurons used an inherited process for neuronal translocation and positioning. In divisions that produced two mitotic daughters, both of which subsequently divided to form four granddaughter cells, only one daughter cell inherited the original basal process while the other extended a new process. Interestingly, behavioral differences were often noted between such mitotic sisters in the trajectory of interkinetic nuclear movement, cell cycle length, and the composition of the granddaughter pair. Therefore, "symmetric" (progenitor --> progenitor + progenitor) divisions are in fact morphologically asymmetric, and the behavior of the mitotic daughters can often be asymmetric, indicating the necessity for studying possible associations between the process inheritance and the cell fate choice.

Calcium signaling to nucleus via store-operated system during cell cycle in retinal neuroepithelium

Intracellular Ca(2+) is a regulatory signal for cell proliferation. To reveal Ca(2+) signal dynamics during cell cycle, we applied Ca(2+) fluorescence imaging to the neural retina of chick embryo, where the soma changes its position during the cell cycle. Purinoceptors were stimulated to cause Ca(2+) release from Ca(2+) stores, since the purinoceptor activation promotes DNA synthesis. Ca(2+) rises occurred in the nucleoplasm of cells at around S-phase. The soma of S-phase cell is located in the inner layer of the retinal neuroepithelium and issues an outer process, which extends to the ventricular surface. Fluorescent probes for endoplasmic reticulum (ER) showed that the ERs in the outer process and the nuclear envelope (NE) or peri-nuclear ER formed the Ca(2+) store. Depletion of the Ca(2+) store induced capacitative Ca(2+) entry (CCE), which caused Ca(2+) rises in the terminal of outer process and soma. The store-operated Ca(2+) signaling declined in M-phase cells and postmitotic cells (retinal ganglion cells (RGCs)) with the loss of the outer process. These results suggest that the Ca(2+) signaling to nucleus via the store-operated system including the ERs in the outer process is crucial for the cell cycle progression in the retinal neuroepithelium.

Intraocular elevation of cyclic AMP potentiates ciliary neurotrophic factor-induced regeneration of adult rat retinal ganglion cell axons

In vitro, cyclic AMP (cAMP) elevation alters neuronal responsiveness to diffusible growth factors and myelin-associated inhibitory molecules. Here we used an established in vivo model of adult central nervous system injury to investigate the effects of elevated cAMP on neuronal survival and axonal regeneration. We studied the effects of intraocular injections of neurotrophic factors and/or a cAMP analogue (CPT-cAMP) on the regeneration of axotomized rat retinal ganglion cell (RGC) axons into peripheral nerve autografts. Elevation of cAMP alone did not significantly increase RGC survival or the number of regenerating RGCs. Ciliary neurotrophic factor increased RGC viability and axonal regrowth, the latter effect substantially enhanced by coapplication with CPT-cAMP. Under these conditions over 60% of surviving RGCs regenerated their axons. Neurotrophin-4/5 injections also increased RGC viability, but there was reduced long-distance axonal regrowth into grafts, an effect partially ameliorated by cAMP elevation. Thus, cAMP can act cooperatively with appropriate neurotrophic factors to promote axonal regeneration in the injured adult mammalian central nervous system.

Postnatal innervation of the rat superior colliculus by axons of late-born retinal ganglion cells

Rat retinal ganglion cells (RGCs) are generated between embryonic day (E) 13 and E19. Retinal axons first reach the superior colliculus at E16/16.5 but the time of arrival of axons from late-born RGCs is unknown. This study examined (i) whether there is a correlation between RGC genesis and the timing of retinotectal innervation and (ii) when axons of late-born RGCs reach the superior colliculus. Pregnant Wistar rats were injected intraperitoneally with bromodeoxyuridine (BrdU) on E16, E18 or E19. Pups from these litters received unilateral superior colliculus injections of fluorogold (FG) at ages between postnatal (P) day P0 and P6, and were perfused 1-2 days later. RGCs in 3 rats from each BrdU litter were labelled in adulthood by placing FG onto transected optic nerve. Retinas were cryosectioned and the number of FG, BrdU and double-labelled (FG+/BrdU+) RGCs quantified. In the E16 group, the proportion of FG-labelled RGCs that were BrdU+ did not vary with age, indicating that axons from these cells had reached the superior colliculus by P0/P1. In contrast, for the smaller cohorts of RGCs born on E18 or E19, the proportion of BrdU+ cells that were FG+ increased significantly after birth; axons from most RGCs born on E19 were not retrogradely FG-labelled until P4/P5. Thus there is a correlation between birthdate and innervation in rat retinotectal pathways. Furthermore, compared to the earliest born RGCs, axons from late-born RGCs take about three times longer to reach the superior colliculus. Later-arriving axons presumably encounter comparatively different growth terrains en route and eventually innervate more differentiated target structures.

Intravitreal injection of adeno-associated viral vectors results in the transduction of different types of retinal neurons in neonatal and adult rats: a comparison with lentiviral vectors

Replication-deficient viral vectors encoding the marker gene green fluorescent protein (GFP) were injected into the vitreous of newborn, juvenile (P14), and adult rats. We tested two different types of modified virus: adeno-associated viral-2-GFP (AAV-GFP) and lentiviral-GFP vectors (LV-GFP). The extent of retinal cell transduction in different-aged animals was compared 7, 21, and 70 days after eye injections. At all postinjection times, LV-GFP transduction was mostly limited to pigment epithelium and cells in sclera and choroid. In contrast, transduction of large numbers of neural retinal cells was seen 21 and 70 days after AAV-GFP injections. AAV-GFP predominantly transduced neurons, although GFP-positive Müller cells were seen. All neuronal classes were labeled, but the extent of transduction for a given class varied depending on injection age. After P0 injections about 50% of transduced cells were photoreceptors and 30-40% were amacrine or bipolar cells. After adult injections 60-70% of transduced cells were retinal ganglion cells. In adults many GFP-positive retinal axons were traced through the optic nerve/tract and terminal arbors were visualized in central targets.

Dendrite development and target innervation of displaced retinal ganglion cells of the chick (Gallus gallus)

The avian accessory optic system (AOS) processes visual signals of translational and rotational flowfields resulting from self-motion. It has been investigated extensively with physiological methods and, because of its anatomical distinction from other retinofugal projections, is well suited for the investigation of dendritic differentiation and axonal pathfinding. Displaced retinal ganglion cells (dRGC) constitute the retinal origin of the AOS. Since little is known about the time course of the development of this projection, we studied the dendritic differentiation of dRGC, their innervation of the nucleus of the basal optic root (nBOR) and the histological development of this target area. dRGC, visualized by retrograde 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate labeling, migrated into the inner nuclear layer of the retina and subsequently developed their characteristic dendritic morphology between E9 and E14. At this stage, dendrites were unistratified in the inner plexiform layer and displayed characteristic branches with 45-90 degrees angles. The frequency of dendritic branches increased from an average of 44 branches per cell at E9 to an average of 155 at E15. This phase was followed by a period of dendritic pruning, E15-E17, where a large number of small branches were eliminated. At the time of hatching, dRGC were morphologically mature with mean dendritic field sizes of 0.28 mm2 and an average of 108 dendritic branches per cell. Retinal innervation of the nBOR occurred between E8 and E11, and tracer injections at later stages revealed no further changes. In addition to the predominant contralateral projection, we have also described a connection to the ipsilateral nBOR. This ipsilateral pathway persisted at least to juvenile stages (P14). The histological development of the nBOR proceeded such that calretinin-immunoreactive neurons were observed from E10 onwards and morphologically described cell types evolved after E12.

Math5 is required for retinal ganglion cell and optic nerve formation

The vertebrate retina contains seven major neuronal and glial cell types in an interconnected network that collects, processes and sends visual signals through the optic nerve to the brain. Retinal neuron differentiation is thought to require both intrinsic and extrinsic factors, yet few intrinsic gene products have been identified that direct this process. Math5 (Atoh7) encodes a basic helix-loop-helix (bHLH) transcription factor that is specifically expressed by mouse retinal progenitors. Math5 is highly homologous to atonal, which is critically required for R8 neuron formation during Drosophila eye development. Like R8 cells in the fly eye, retinal ganglion cells (RGCs) are the first neurons in the vertebrate eye. Here we show that Math5 mutant mice are fully viable, yet lack RGCs and optic nerves. Thus, two evolutionarily diverse eye types require atonal gene family function for the earliest stages of retinal neuron formation. At the same time, the abundance of cone photoreceptors is significantly increased in Math5−/− retinae, suggesting a binary change in cell fate from RGCs to cones. A small number of nascent RGCs are detected during embryogenesis, but these fail to develop further, suggesting that committed RGCs may also require Math5 function.

Development of the visual system of the chick. I. Cell differentiation and histogenesis

This review summarizes present knowledge on the embryonic development of the avian visual projections, based on the domestic chick as a model system. The reductionist goal to understand formation and function of complex neuroanatomical systems on a causal level requires a synthesis of classic developmental biology with recent advances on the molecular mechanisms of cell differentiation and histogenesis. It is the purpose of this article. We are discussing the processes underlying patterning of the anterior neural tube, when the retina and optic tectum are specified and their axial polarity is determined. Then the development of these structures is described from the molecular to the anatomical level. Following sections deal with the establishment of secondary visual connections, and the developmental interactions between compartments of the retinotectal system. Using this latter pathway, from the retina to the optic tectum, many investigations aimed at mechanisms of axonal pathfinding and connectivity have accumulated a vast body of research, which will be covered by a following review.

Partial rescue of the ocular retardation phenotype by genetic modifiers

The or(J) allele of the murine ocular retardation mutation is caused by a premature stop codon in the homeodomain of the Chx10 gene. When expressed on an inbred 129/Sv strain, the or(J) phenotype is characterized by microphthalmia and a thin, poorly differentiated retina in which the peripheral portion is affected to a greater extent than the central portion. Such mutant retinae lack differentiated bipolar cells and the optic nerve typically fails to form, leading to blindness. Here, we show that progeny from an outcrossed backcross between 129/Sv-or(J) /or(J) and Mus musculus castaneus produce animals that are homozygous for the or(J) mutation and exhibit a much ameliorated eye phenotype. Although not of normal size, such modified or(J) eyes are significantly larger than those in 129/Sv-or(J) /or(J) mice, and contain a better organized retina which includes bipolar cells. Furthermore, optic nerves are frequently present, and the eyes show a degree of function as reflected by electroretinogram and pupillary response. As in 129/Sv-or(J) /or(J) mice, however, modified or(J) eyes show incomplete growth and a lack of cell differentiation in the periphery of the retina. The selective, and apparently nonmodifiable, effect of the ocular retardation phenotype on the periphery of the retina indicates that Chx10 plays an important role in the central-to-peripheral gradient of retinal development. These findings demonstrate that the ocular retardation phenotype can be greatly modified by the genetic background, and help to define a role for Chx10 in ocular development.

Survival and process regrowth of purified chick retinal ganglion cells cultured in a growth factor lacking medium at low density. Modulation by extracellular matrix proteins

Panning-purified retinal ganglion cells (RGCs) were cultured at low density in a chemical-defined growth factor (GF)-lacking medium on substrate of different extracellular matrix (ECM) proteins. The process regrowth under these severe conditions were evaluated by morphometric measurements and by cell ELISA (CELISA) performed for neurofilaments regardless of their phosphorylated state (NF-CELISA), or for phosphorylated neurofilaments (PNF-CELISA), to respectively assess process regrowth or axonal development. The development obtained in cultures performed on laminin was taken as standard to refer the other substrata. The cellular content of Thy-1 required for panning purification as well as the gangliotetraosylganglioside (GTOG) expression and the lack of the immunolabeling of the RA4 antigen strongly suggest that the purified RGCs were mature neurons. About 80% of the 7-day-old embryo (E7)-RGCs survived 4 days in culture on any substrate, including polylysine. Conversely, E10-RGCs in about 75% of cultures on polylysine did not survive for 4 days. E7-RGCs developed better on thrombospondin and vitronectin. E10-RGCs cultured on vitronectin grew better than on laminin; on thrombospondin and collagen, E10-RGCs grew like on laminin and on fibronectin they had a poor development. The values of PNF-CELISA obtained on vitronectin, collagen and fibronectin on E7-RGC cultures were significantly higher than on laminin, which are in agreement with the longer processes observed. The flavoridin disintegrin caused a dose-response inhibition on E7-RGC cultures on thrombospondin but not on laminin, suggesting on process regrowth, the integrin-thrombospondin interaction(s) are significantly involved, while on laminin, it is the non-integrin receptor(s) which are significant involved.

Embryonic lens repels retinal ganglion cell axons

During development of the vertebrate visual system, retinal ganglion cell (RGC) axons follow a precise path toward their midbrain targets. Although much is known about the cues that direct RGC axons once they have left the optic disc, less is known about the guidance of axons at earlier stages, when RGCs first send out their axons to navigate within the developing retina. Using collagen gel coculture experiments, we find that the embryonic lens produces a powerful diffusible repulsive activity for RGC axons. We also find that this activity is localized to the lens epithelium and not the lens fiber layer, while the pigmented epithelium and vitreous humour are devoid of activity. The further observation that the lens also chemorepels primary sensory axons, but does not repel olfactory bulb axons, shows that this activity is specific for subsets of axons. Our experiments have excluded two candidate repellents for RGC axons (collapsin-1/sema III and chondroitin sulfate proteoglycans). These results implicate the lens in the earliest stages of RGC axon guidance. One function of the lens repellent may be to prevent aberrant targeting toward the lens, and it may also be involved in the directional guidance of RGC axons toward the optic disc.

Positional cloning of the gene for X-linked retinitis pigmentosa 2

X-linked retinitis pigmentosa (XLRP) results from mutations in at least two different loci, designated RP2 and RP3, located at Xp11.3 and Xp21.1, respectively. The RP3 gene was recently isolated by positional cloning, whereas the RP2 locus was mapped genetically to a 5-cM interval. We have screened this region for genomic rearrangements by the YAC representation hybridization (YRH) technique and detected a LINE1 (L1) insertion in one XLRP patient. The L1 retrotransposition occurred in an intron of a novel gene that consisted of five exons and encoded a polypeptide of 350 amino acids. Subsequently, nonsense, missense and frameshift mutations, as well as two small deletions, were identified in six additional patients. The predicted gene product shows homology with human cofactor C, a protein involved in the ultimate step of beta-tubulin folding. Our data provide evidence that mutations in this gene, designated RP2, are responsible for progressive retinal degeneration.

Maturational changes in cell surface antigen expression in the mouse retina and optic pathway

The distribution of the cell surface molecules M6 and L1 was studied using the immunohistochemistry and in situ hybridization in the developing and adult mouse retina and optic nerve. L1 is a cell adhesion molecule while M6 is a cell surface molecule homologous to the myelin protein proteolipid protein (PLP/DM20). Although both molecules were expressed in retina and optic nerves of embryonic and neonatal mice, our studies show that their patterns of postnatal expression are quite different. While L1 continues to be expressed in optic axons throughout adulthood, expression of M6 on optic axons declines after birth and instead becomes strongly expressed on Müller glial endfeet and in the inner plexiform layer. The modulation of these molecules after birth could provide clues to changing cell-cell interactions occurring in the proximal portion of the optic pathway.

Axonal versus dendritic outgrowth is differentially affected by radial glia in discrete layers of the retina

Formation of neural cell polarity defined by oriented extension of axons and dendrites is a crucial event during the development of the nervous system. Ganglion cells of the chicken retina extend axons exclusively into the inner retina, whereas their dendrites grow into the outer retina. To analyze guidance cues for specific neurite extension, novel in vitro systems were established. Ganglion cells were purified by enzymatically facilitated detachment of the ganglion cell layer. A newly developed retrograde labeling technique and the expression analysis of the cell type-specific 2A1 antigen were used to monitor ganglion cell purification. In highly purified ganglion cells explanted onto retinal cryosections (cryoculture), axon formation was induced when the cells were positioned on the inner retina. In contrast, on outer layers of the developing retina dendritic outgrowth was prevalent. Because radial glia have been demonstrated to be instructive in neuritogenesis, distinct glial cell compartments located in inner and outer retina, respectively, were isolated for functional assays. Glial end feet were purified by a physical detachment technique. Glial somata were purified by complement mediated cytolysis of all nonglial cells. When ganglion cells were cultured on different glial compartments, axon formation occurred on end feet but not on glial somata. In striking contrast, on glial somata dendrites were formed. The data support the notion that ganglion cell polarity is affected by the retinal microenvironment, which in turn is possibly influenced by radial glia, being themselves polarized.

Chiasmatic specificity in the regenerating mammalian optic nerve

The mammalian central nervous system is capable of regenerating; however, there is no evidence that the regenerating axons can navigate along their normal pathways and reestablish topographically organized projections: essential for functional return of vision. Here retinal ganglion cells in the opossum Monodelphis were birthdated with tritiated thymidine on the sixth postnatal day (P6), before being lesioned in the temporal retina at P8. Retrograde tracing with horseradish peroxidase injected into the ipsilateral optic tract at P24 showed that the temporal crescent had reformed behind the retinal lesion. By comparisons of cell and thymidine counts from lesioned and control regions of retina, it was estimated that about 40% of the normal number of ganglion cells are able to regenerate into the ipsilateral optic tract following a lesion in the temporal retina at P8. A clear line of decussation (separation of ipsilateral and contralateral projections) reformed in the lesioned temporal retina and regenerating ganglion cells labeled with DiI were turned at appropriate points on passing through the optic chiasm. This is evidence of chiasmatic specificity with regard to lesioned retinal ganglion cells regenerating into the ipsilateral optic tract.

Survival and differentiation of purified embryonic chick retinal ganglion cells cultured at low density in a chemically defined medium

Both, a tailored chemically defined nutrient medium (BP5) and a sandwich culture sustain the survival for more than a week and allow the differentiation of embryonic chick retinal ganglion cells (RGCs) seeded at low density. Purification of RGCs from 7-11-day old embryos was accomplished by panning using specific anti-chicken Thy-1 antibodies immobilized in plaques. Yield of RGCs was less than 1% of the calculated number of these cells in the used retinas. This result agrees with the scarce expression of Thy-1 in immature retina; accordingly, the most mature RGCs are those probably selected by the panning. This assumption obtained support on the expression of gangliotetraoxylgangliosides (GTOG), that characterize the differentiated retinal neurons. Thus, the outgrowth of processes observed in cultured cells, might imply axonal regeneration in mature neurons. This manageable RGC culture method approaches a system for studying the in vitro trophic factors and substrata which affect axonal regrowth in central nervous system cells.

Growth cone interactions with purified cell and substrate adhesion molecules visualized by interference reflection microscopy

The migration of growth cones on substrates consisting of naturally occurring cell adhesion molecules has been extensively studied in cell culture. However, relatively little is known about how growth cones contact the substrate or how the patterns of contact change as growth cones move forward. We have examined the interactions of chick retinal ganglion cell growth cones with laminin, merosin, N-cadherin, L1 and poly-L-lysine by time-lapse interference reflection microscopy (IRM) using a laser scanning confocal microscope. In images obtained by IRM, areas of a cell that are closely apposed to the substrate appear dark whereas areas that are farther away appear light. Growth cones on Jaminin and merosin were almost uniformly light, indicating that very little of the membrane was in close contact with the substrate. Growth cones on N-cadherin had a mottled appearance with some relatively large dark gray areas. The proximal portions of filopodia often were dark, in contrast to those on laminin and merosin which were light. In addition, growth cones on N-cadherin had numerous dark gray punctate regions of close association with the substrate. Growth cones on L1 had darker regions than growth cones on other substrates and these comprised a larger fraction of their area. There also were differences in the temporal dynamics of growth cone interactions with different substrates and these differences correlated with differences in rates of growth. None of the contacts observed in growth cones were as dark or stable as focal contacts of fibroblasts.

Retinal ganglion cell dendritic development and its control. Filling the gaps

The way in which central neurons acquire their complex and precise dendrite arbors is of considerable developmental interest. Using retinal ganglion cells (RGCs) as a model, the mechanisms that pattern dendritic development are beginning to emerge. As in other systems, final dendrite phenotype is achieved by a mixture of intrinsic and extrinsic determinants. The extrinsic determinants of RGC dendrite shape reflect the anatomical constraints of producing a paracrystalline mosaic of arbors that laminates the inner plexiform layer of the retina. In this article, the key features of RGC dendrite development are reviewed. The emerging molecular mechanisms behind dendritic laminar segregation and "dendritic competition" are described. The role of afferent extrinsic influences are contrasted with those of retrograde, activity-dependent target influences that may regulate the final maturational phase of dendrite remodeling.

Comparative localisation of CRYP alpha, a CAM-like tyrosine phosphatase, and NgCAM in the developing chick visual system

The avian CRYP alpha gene is expressed in the embryonic nervous system and encodes a receptor-like protein tyrosine phosphatase with structural similarity to neural cell adhesion molecules. To gain further insight into the role of the CRYP alpha phosphatase in neural development, this study addresses the protein's cellular distribution in the well characterised embryonic visual system. High levels of CRYP alpha protein localise in retinal axons extending from the eye to the tectum throughout the major growth periods of these nerve processes. In addition, primitive inner plexiform layer processes in the retina, tectobulbar axons, and non-retinal fibres of the tectal stratum opticum, contain large amounts of CRYP alpha. Its presence in non-fasciculated processes suggests that CRYP alpha has a role other than in fasciculation in short range fibres. In contrast to CRYP alpha, NgCAM is confined largely to axon fascicles in the retina and tectum, consistent with its demonstrated role in fasciculation of cultured neurites. In cultured retinal neurons CRYP alpha proteins reside both in neurite processes and in growth cone membranes, implicating both of these as potential functional locations for the protein. Although CRYP alpha continues to be expressed in the later embryo, the strong, early expression suggests a significant developmental role in the initial growth or guidance of nerve processes. This applies both over the longer range in the retinotectal and tectobulbar projections and over the shorter range within plexiform layers.